1. Field of the Invention
The present invention relates to a lead feed mechanism for a mechanical pencil, and particularly to a lead feed mechanism for a mechanical pencil wherein a top-pressing lead feed mechanism, a bottom-pressing lead feed mechanism and an automatic lead feed mechanism are provided so that a lead can be automatically fed by stopping pushing the lead onto the surface of writing paper.
2. Description of the Prior Art
Generally, the top of the body of a conventional mechanical pencil is pressed to protrude a lead by a desired length from a fixed lead pipe at the bottom of the mechanical pencil to perform writing. When the bottom of the lead is worn due to the writing, the writing is stopped and the top of the body of the pencil is pressed again to feed the lead to resume the writing. Such operation is repeated to enable the writing for a long period of time. However, the improvement of the lead feed mechanism of the mechanical pencil of the top-pressed type has been desired for the reasons why the fixed lead pipe scratches the surface of writing paper due to the wear of the lead and the finger-held position of the pencil needs to be changed in order to press the top of the pencil to feed the lead.
In order to perform the improvement of the lead feed mechanism of the mechanical pencil of the top-pressed type, various lead feed mechanisms of the bottom-pressing type, wherein the bottom of a mechanical pencil is once pushed onto writing paper or the like and the pushing of the bottom onto it is then stopped so that a lead is fed from the bottom of a movable lead pipe provided in a slider at the bottom of the pencil, have been proposed. A mechanical pencil, which was disclosed in the Japanese Patent Laid-open Gazette No. 57-188399, has one of the conventional lead feed mechanisms of the bottom-pressing type.
The conventional mechanical pencil of the bottom-pressed type has a construction as shown in FIG. 1. A bottom member 2 is screwed to the bottom of an outer pipe 1, which is held by the fingers of a writing person. The top of the outer pipe 1 is screwed to a nut 3a secured to the bottom of an upper pipe 3 containing a lead pipe 4. A cylindrical member 20 is fixed in the bottom member 2. A slider 21, to which a lead protection pipe 22 is secured at the bottom of the slider, is provided in the cylindrical member 20 so that the slider is slidable in the axial direction of the mechanical pencil. The slider 21 has a large-diameter portion and a small-diameter portion continuous to the step 21a of the large-diameter portion. A plurality of balls 23 are rotatably supported on the large-diameter portion. A friction member 24, which is made of rubber, synthetic resin or the like and has a nearly cylindrical form and whose inside circumferential surface is placed with a prescribed frictional force on the outside circumferential surface of a lead 5, is provided in the hole 21b of the slider 21, which is opened at the top of the slider. A first spring 25 is provided near the top of the cylindrical fixed member 20. The inside circumferential surface of the member 20 in the bottom member 2 has an inside circumferential groove 20b, which holds the balls 23 for the slider 21. A bottom bearer 6 is fitted in the bottom of the lead pipe 4. The outside circumferential surface of the bottom bearer 6 is fitted with a fixed pipe 7 projecting down from the bottom of the bearer 6. A slidable sleeve 10 is provided in the fixed pipe 7. A lead chuck 11, which is split in plural, for example, two parts to pinch the lead 5 on both sides, is provided in the sleeve 10. The thick bottom portion of the lead chuck 11 has recesses 11a. The bottom portion of the sleeve 10 has a tapered inside circumferential surface 10a whose diameter increases downwards. Balls 12 for causing the lead chuck 11 to pinch and release the lead 5 are provided between the tapered inside circumferential surface 10a and the recesses 11a. A circumferential projection 10b is provided at the bottom of the outside circumferential surface of the sleeve 10. A movable pipe 13 is provided over the circumferential projection 10b. A second spring 14 is provided between a flange 13a projecting from the top of the movable pipe 13 and a flange 6a provided on the outside circumferential surface of the central portion of the bottom bearer 6. A slider restriction pipe 15, to which an annular plate 15a is secured as a slider pusher at the bottom of the pipe 15, is slidably supported on the outside circumferential surface of the movable pipe 13. The top 15b of the slider restriction pipe 15 can be engaged with the flange 13a at the top of the movable pipe 13. A flange 15c provided in a prescribed position on the outside circumferential surface of the slider restriction pipe 15 near its bottom can be engaged with the bottom 1a of the outer pipe 1. A spring bearer 11b projects in an optional position from the outside surface of the top portion of the lead chuck 11. A step 10c is provided next to the smallest-diameter section of the tapered inside circumferential surface 10a of the sleeve 10. A third spring 16 is provided between the step 10c and the spring bearer 11b.
The operation of the conventional mechanical pencil of the bottom-pressed type, which has the construction described above is hereafter described. In the first step of the operation, the lead 5 protruding by a length of X down from the lead protection pipe 22 is pushed onto the surface P of writing paper, as shown in FIG. 1. Since the lead is supported by the friction member 24 fitted in the slider 21 and is pinched by the lead chuck 11 under a pinching force applied from the tapered inside circumferential surface 10a of the sleeve 10 through the balls 12, the slider, the lead chuck, the sleeve and the movable pipe 13 engaged with the circumferential projection 10b of the sleeve 10 follow together the lead 5 by the length of X.
In the second step of the operation, the writing person pushes the bottom of the mechanical pencil onto the surface P of the writing paper so that the lead 5 and the lead protection pipe 22 are moved down together in the outer pipe 1 and the bottom member 2. At the same time, the lead chuck 11 pinching the lead 5, and the sleeve 10, the movable pipe 13 and so forth following the lead chuck are moved up by a length of L1 against the elastic forces of the second and the third springs 14 and 16 so that the slider restriction pipe 15 following the movable pipe due to a sliding frictional resistance is moved following the movement of the lead chuck, the sleeve, the movable pipe and so forth. The balls 23 for the slider 21 are then engaged into the inside circumferential groove 20b of the cylindrical fixed member 20 as the distance of D1 between the top 21a of the slider 21 and the bottom of the annular plate 15a is kept constant, as shown in FIG. 2. As a result, the movement of the slider 21 is first stopped, and the flange 15c of the slider restriction pipe 15 following the lead chuck 11 is then engaged with the bottom 1a of the outer pipe 1 so that the movement of the slider restriction pipe is also stopped. Although the movement of the slider restriction pipe 15 is stopped, the movable pipe 13 and the sleeve 10 are moved by a length of L1-D1 as shown in FIG. 2, so that a gap of D2 corresponding to the length of L1-D1 is made between the flange 13a of the movable pipe 13 and the top 15d of the slider restriction pipe 15.
In the third step of the operation, when the writing person stops pushing the bottom of the mechanical pencil onto the surface P of the writing paper, the lead chuck 11 is moved down by the distance of D1 together with the sleeve 10, the movable pipe 13 and the slider restriction pipe 15 by the elastic force of the third spring 16 between the step 10c of the sleeve 10 and the spring bearer 11b of the lead chuck 11 as the lead chuck remains pinching the lead 5, so that the bottom of the annular plate 15a and the top 21a of the slider 21 come into contact with each other. As a result, the lead chuck 11 releases the lead 5 so that the lead is thrusted by a length corresponding to the gap D2, as shown in FIG. 3.
In the fourth step of the operation, the annular plate 15a urges the top 21a of the slider 21 downwards so that the balls 23 are moved out of the inside circumferential groove 20b of the cylindrical fixed member 20. As a result, the slider 21 is moved down by the length of L1 due to the elastic force of the first spring 25 so as to protrude the lead protection pipe 22 downwards by the length of L1 and to move down the lead 5 by the length of L1 until the bottom 10c of the sleeve 10 comes into contact with the inside circumferential flange of the slider restriction pipe 15. After all, the lead 5 is protruded from the bottom of the lead protection pipe 22, by a length corresponding to the gap of D2, so that a lead portion having a length of L2 and usable for writing is thrusted.
However, the above-described conventional mechanical pencil of the bottom-pressed type has various problems mentioned below.
Since an engagement mechanism comprising the balls 23 and the inside circumferential groove of the cylindrical fixed member 20 is used as a means for holding the slider 21 on the cylindrical fixed member 20 until the slider begins to be returned downwards after the slider provided in the bottom member 2 is moved up, a holding force necessary to perform the above-described ideal operation cannot be obtained, so that the lead 5 cannot be surely thrusted, and at the worst, the lead cannot be thrusted at all by pressing the bottom of the mechanical pencil. This is the first of the problems.
With the above-described operation, even if the pinching force is removed from the lead chuck by the sleeve 10 and the balls 12 in order to feed the lead 5, the lead chuck cannot surely stop pinching the lead, so that the lead cannot be thrusted downwards by the length corresponding to the gap of D2. This is the second of the problems.
Although the top of the lead chuck 11 is restrained by the inside circumferential surface of the sleeve 10 so as not to spread, a means for preventing the lead chuck from being compressed is not provided, so that the inside circumferential surface of the sleeve 10 sometimes causes the spring bearer 11b of the lead chuck 11 to play inwards due to a complicated action so as to spread the lead-pinching bottom portion of the lead chuck. For that reason, the pinching force of the lead chuck 11 on the lead 5 is reduced so as to hinder thrusting the lead. This is the third of the problems.
The gap between the open edge of the bottom 10b of the sleeve 10 and the lead chuck 11 is possibly increased by the balls 12 so that the balls drop out of the recesses 11a during the lead feeding operation. Once the balls 12 have dropped out, they cannot be fitted again in the complicated mechanical pencil. Therefore, the reliability of the mechanical pencil is low. This is the fourth of the problems.
Though the movable pipe 13 and the slider restriction pipe 15 are slid together in such a manner that they follow each other due to the sliding frictional resistance between the outside circumferential surface of the movable pipe and the inside circumferential surface of the slider restriction pipe, the sliding frictional resistance cannot be obtained to a prescribed degree when the outside and the inside circumferential surfaces of the movable pipe and the slider restriction pipe are worn due to the long-period use of the mechanical pencil, so that the lead cannot be thrusted by pressing the bottom of the pencil. This is the fifth of the problems.
Since the sleeve 10, the movable pipe 13 and the slider restriction pipe 15 are triply provided around the lead chuck 11 for pinching the lead 5 and housed in the outer pipe 1, the whole mechanism of the mechanical pencil is complicated, and it is difficult to make the pencil thin, so that it is hard to meet the requirement of reducing the size and weight of the pencil. This is the sixth of the problems.